1. Field Of The Invention
The invention relates to video/data projection systems, and more particularly to a full color video/data projection system employing a single monochrome thin film transistor liquid crystal panel.
2. Description of the Related Art
The predominant consumer rear screen projection systems in use today for diagonal screen sizes of more than 40 inches are commonly known as cathode ray tube, or CRT projection televisions. Such systems utilize three projection CRTs, one for each primary color, red, green and blue to produce a full color image.
FIG. 1A is a side view of a conventional rear screen projection display using 3 CRT""s. The system is contained in a display cabinet 12 with a viewing screen 14. Viewing screen 14 may be, for example, 60 inches measured diagonally. An image 18 is formed by a CRT 16 in the lower portion of display cabinet 12. The CRT projection lens (located just in front of the CRT faceplate) reforms the image at viewing screen 14 by reflecting the CRT light from mirrors 20 and 22. Viewing screen 14 is made of an appropriate material such that a viewer may see an image from the front side of the viewing screen 14, when the screen is illuminated from the backside.
FIG. 1B is a top view of the lower portion of the rear screen projection system within display cabinet 12 containing the three CRTs. The three CRT/projection lens assemblies 16r, 16g, and 16b are positioned so their respective images 18r, 18g, and 18b will overlap as they strike the viewing screen.
The size and weight of the three CRT""s make the projection system rather large and heavy. In addition, for systems where the diagonal of the viewing screen is on the order of 50 to 70 inches, the brightness requirement for the CRT""s becomes very large. The brightness requirement in turn limits the resolution of the projection system to the extent that many CRT systems have difficulty supporting resolutions greater than the resolution standard imposed by the National Television Standards Committee (NTSC).
Therefore, for high definition television (HDTV) resolutions, such as 1280 by 1080 pixels interlaced (1080i) or 720 pixels progressive (720p), projection systems are turning to digital light processing (DLP) and liquid crystal display (LCD) technologies. A rear screen projection system utilizing LCD or DLP technology is configured similar to the CRT arrangement shown in FIGS. 1A and 1B, in that a lamp and the image forming device(s) are situated in the lower portion of the display cabinet with mirrors and projection lenses arranged to magnify the image from the image forming device to the viewing screen.
DLP technology utilizes an image forming reflective chip (a DLP chip) with microelectromechanical mirrors constructed thereon. Light is directed onto the DLP chip and the DLP chip selectively modulates the light reflected from the chip by tilting the micro-mirrors. The reflected light passes through a projection lens assembly to form a display image. To produce a full color image, a DLP projector uses a spinning color wheel at the light source operated in accordance with the conventional frame sequential color method.
The DLP rear screen projection system has several disadvantages. First, the DLP chip is an expensive component and, in an effort to minimize the cost, the chip is generally manufactured in small dimensions, for example, a diagonal of 0.7 inches. The small chip dimension in turn requires complicated and costly projection lenses because of the large magnification required, from less than one inch up to 40 to 70 inch diagonal screens, and because of the short throw distance available, typically around 3 feet, within the television display cabinet. Also, the small chip dimension requires a very short arc in order to efficiently utilize the lamp light. Very short arc lamps have short lamp life and replacement cost is high. Additionally, moving parts necessary for controlling the spinning color wheel present reliability problems and system noise.
LCD projectors can use either transmissive or reflective liquid crystal display panels as the image forming elements. Reflective LCD panels are fabricated on silicon substrates and therefore, in order to control costs, have similar dimensions to DLP chips, typically less than one inch diagonal. The reflective Liquid Crystal on Silicon (LCOS) panels used in rear screen projection systems typically employ a frame sequential method for producing a full color image. Therefore, LCOS system have all the same disadvantages of DLP systems.
LCD projectors using transmissive liquid crystal display panels as the image-forming element typically use panels fabricated on a quartz polysilicon substrate. Again, to control costs, these panels usually have a diagonal dimension of less than 1.3 inches. In an LCD projector, polarized light is transmitted through an LCD panel, typically a thin film transistor (TFT) LCD panel. The LCD panel modulates the polarization of the light according to the desired image data. The modulated light then passes through another polarizer, or an analyzer, to form the desired image. The image is magnified and reformed for display on a viewing screen.
The polysilicon LCD rear screen projection systems can use one to three small LCD panels. Generally, for full color display, the three panel configuration is used, one panel for each primary color channel. The three panel configuration is preferred because the light efficiency of the LCD panels is not as high as DLP and because the LCD panels generally available cannot modulate the light as fast as a DLP device. A further disadvantage of using three LCD panels is that they require complex assembly techniques to optically align the LCD""s such that the different colored pixels of each panel are precisely aligned in the final image. Ensuring such alignment over time presents a further complexity for the system. Again, the projection lens of an LCD rear screen projector further requires a large magnification within a short throw distance and therefore increases the cost and complexity of the system.
Single panel LCD rear screen projection systems have been used for projection screens of 40 to 60 inches measured diagonally. The LCD panels used in these systems typically range from 6 to 18 inches in diagonal. Each full color pixel within the panel is divided into separate sub-pixels that are dyed or colored red, green, and blue. This method of producing full color has the disadvantage of severely reducing the transmission of the panel because of the properties of the dye material and the loss of spatial resolution. The combined effect of these two factors reduces the percentage of light transmitted through a panel to between 4 and 6 percent, approximately one-sixth the amount of an equivalent panel without dye material. In an attempt to increase the transmission and thus the brightness of the image produced, the colors are de-saturated, i.e., the red dye is made lighter or less vivid in order to transmit more light. Color de-saturation, as expected, affects the quality of the image projected. Therefore, the single panel LCD rear screen projection system suffers from both low system brightness and poor color quality absent additional lamps and optics.
It is therefore desirable to provide a rear screen projection system with high resolution and brightness without complicated and expensive lens assemblies. Thus, a rear screen projection system capable of generating an efficient high quality full color display image with reduced manufacturing and component costs is desired.
According to one embodiment of the present invention, an apparatus for projecting an image includes an illumination source generating polarized light beams in three different primary colors and a monochrome liquid crystal display panel including a two-dimensional array of addressable pixels. The array of addressable pixels is divided into multiple image areas, each image area receiving one of the polarized light beams in a respective primary color and modulating the polarized light beam to generate an image in the respective primary color. The apparatus further includes a projection lens assembly for aligning and projecting the images generated by the multiple image areas of the liquid crystal display panel to form a full color image.
In one embodiment, the illumination source includes a light source emitting randomly polarized white light, a polarizing element coupled to receive the randomly polarized white light and generate a polarized white light beam, and color separating elements for separating the polarized white light beam into polarized light beams in three different primary colors.